Construction on the freeway began soon after the completion of the Don Valley Parkway, with the first section south of Steeles Avenue opening in 1977. Over the next twelve years, the Ministry of Transportation of Ontario (MTO) undertook a continuous construction program to build the freeway to Davis Drive in Newmarket. This was completed on 24 October 1989. The route has undergone a periodic series of smaller extensions and widening in the years since, now travelling a further 15.5 km (9.6 mi) north to Woodbine Avenue near Ravenshoe Road in the town of East Gwillimbury. Future proposals may one day result in an extension to southeast of Beaverton.

Highway 404 is one of several freeways in the Greater Toronto Area (GTA) with High-Occupancy Vehicle (HOV) lanes; the southbound lane was one of the initial projects in the province and opened on 13 December 2005. The northbound lane opened on 23 July 2007.

Running parallel to Highway 400 approximately 15 kilometres (9.3 mi) to the east, Highway 404 extends 50.1 km (31.1 mi) on a north–south orientation between Highway 401 and Woodbine Avenue. There are 16 interchanges along its length, mostly of the Partial cloverleaf A4 configuration. Exit numbers on the freeway start at 17, suggesting that the length of the Don Valley Parkway was considered in distance calculations; until 2017, there were no exit numbers posted on the DVP.[2][3]

Officially, the Ministry of Transportation of Ontario (MTO) jurisdiction over the freeway begins as the opposing directions of travel diverge south of the Highway 401 interchange.[note 1]
Northbound, two lanes from the DVP are joined by a third from the eastbound collectors of Highway 401. These narrow to two lanes before merging with a single lane from westbound Highway 401 immediately south of Sheppard Avenue. An additional two lanes from westbound Highway 401 converge and form a separate carriageway with no access to Sheppard.[4]
Southbound, the freeway is divided into two carriageways, both of which provide access to the DVP. The outer carriageway also provides access from Sheppard and to Highway 401, including the westbound express lanes, while the inner carriageway is intended for DVP-bound traffic. The HOV lane also provides access to westbound Highway 401 via a dedicated tunnel, which passes beneath the other southbound lanes.[5]
To the east of Highway 404 is the Consumers Road office park. To the west and north of Sheppard Avenue is Fairview Mall, which has its own connection with the southbound lanes attached to the Sheppard interchange.[2]

The highway continues directly north along the old Woodbine Avenue right-of-way to just south of Steeles Avenue, where it diverges to the west before continuing north. From just north of Sheppard, a northbound HOV lane is present alongside the central median. Southbound, the HOV lane continues as far as Highway 401. Alongside Highway 404 to the east is an industrial warehouse and commercial office area, while on the west is a suburban subdivision of North York. Northbound, the freeway is six lanes wide from Sheppard Avenue to Finch Avenue, where one lane diverges onto an off-ramp, re-emerging north of Finch. Southbound, it is six lanes wide from Steeles south to Sheppard.[2]

Highway 404 north of Toronto

At Steeles Avenue, the freeway enters the Regional Municipality of York. To the east are industrial units, while on the west are residential suburbs. This land-use persists north to the Highway 407 ETR interchange, a multi-level stack interchange with two flyovers. North of Highway 407, the freeway crosses Highway 7, where the HOV lanes transition to standard through lanes. The freeway passes west of Buttonville Municipal Airport and then interchanges with 16th Avenue. It narrows and the central barrier ends; a grass median taking its place between the opposing lanes. The land-use density continues to drop, with the appearance of some open spaces and farms interspersed with industrial and commercial buildings. By 19th Avenue, just north of the Honda Canada headquarters in Markham, the land-use is agricultural on both sides of Highway 404. Highway 404 continues north, forming the eastern boundary of the municipalities of Richmond Hill, Aurora and Newmarket and the western boundary of Whitchurch-Stouffville. North of Wellington Street, the highway reduces in width to four lanes, which is its configuration north through East Gwillimbury.[6] The route continues, passing east of the community of Sharon, eventually curving northeast and terminating at an at-grade intersection with Woodbine Avenue immediately south of Ravenshoe Road (York Regional Road 32).[2]

A freeway east of Highway 11 was planned as early as 1954, when the province extended Highway 48 south from Port Bolster. A large cloverleaf interchange was constructed with the Toronto Bypass, and plans formulated for a dual highway around the east side of Lake Simcoe, connecting with Highway 11 near Orillia or Gravenhurst. This route was dropped when Metropolitan Toronto began planning for the northern extension of the DVP in 1957, as subdivisions encroached upon Woodbine Avenue north of Highway 401. The six-lane expressway was to follow the alignment of Woodbine from its southern terminus at Lawrence Avenue to north of Steeles Avenue, where the Department of Highways (DOH) would continue the road as a "new King's Highway".[3]

Facing south along Woodbine Avenue in 1965; the crossroad is Finch Avenue, which passes over the ten lane freeway that has since replaced this rural road

In 1959, the DOH announced that they would construct and maintain the new route once the DVP was completed to Highway 401 and designate it Highway 404.[7]
The proposed route of the freeway was presented at a special delegation on 13 December 1960 by Harold Barry, a representative of the department.[8]
Design work started in 1973,[9] and construction began following the awarding of a C$6.9 million contract in March 1976. This contract included construction of the Finch Avenue interchange, overpasses at McNicoll and Van Horne Avenues and 4.5 km (2.8 mi) of six-lane freeway between Sheppard and Steeles Avenues.[10][11]
Shortly thereafter, on 20 April, Ernest Avenue and Van Horne Avenue were closed to traffic at Woodbine.[12]
The first section of Highway 404 between Highway 401 and Steeles Avenue opened in late 1977, including the flyover ramp from southbound Woodbine Avenue.[13]
The freeway was separated by a grass median with a steel beam acting as a barrier between the lanes. Construction north of Toronto proceeded quickly, with the contract for the section from Steeles to Highway 7 being awarded in 1976[13] and the section opening on 10 November 1978.[14]
The next extension, to Stouffville Road (then known as the Gormley Side Road), was opened ceremoniously on 9 December 1980 by minister James Snow;[15]
the segment north of Highway 7 was four lanes wide.[16]

Highway 404 looking southbound from Finch Avenue. Note the HOV lane on the left side of the carriageway.

The section of Highway 404 north of Stouffville Road was the subject of considerable controversy when work began to clear the route on 15 May 1981 before the completion of an environmental impact assessment. The Ministry of Transportation and Communications was charged with violating the newly enacted Environmental Assessment Act, which it contested came into effect after construction of the Highway 404 extension had begun.[17]
Minister James Snow was charged with violating the act, and called upon to resign. The Minister did not resign, but paid a $3,500 fine. Despite the issues surrounding it, the extension between Stouffville Road and Bloomington Road was opened ceremoniously on the morning of 10 August 1982.[18]

Construction on the segment north of Bloomington to Aurora Sideroad was already in progress by this point. It was opened to traffic in late September 1985.[19]
Construction on the 6.5 km (4.0 mi) section from Wellington Street to Davis Drive began in early 1986, and the section opened to traffic on 24 October 1989 at 8:30 am. This final segment cost $22.1 million, ending the continuous construction program undertaken since 1973 at a cost of $83.3 million.[9]

Studies and environmental assessments into various extensions began almost immediately after the completion of the route to Newmarket in 1989; it would take over a decade for any northward progression to take place. The completion of the route to Davis Drive was met with scorn as traffic in Newmarket rapidly increased as the bedroom community grew with the new highway access. Municipal officials warned prior to the opening of the route that major traffic delays would be faced along Davis Drive. Then-mayor Ray Twinney began an immediate push to widen Green Lane – at that time an unpaved rural route – into a bypass of the town.[20]
Traffic delays were also compounded at the southern end of Toronto, where drivers whom had previously made use of Yonge Street, Bayview Avenue or Leslie Street would shift to make use of the new freeway.[21][22]

By 1992, York Region was moving forward with plans to expand Green Lane into a four lane road, while the province was urged to consider extending Highway 404 north to it, and eventually around the east side of Lake Simcoe.[22] The province studied this and other options over the following years, before a formal announcement was made by Minister of Transportation Tony Clement on 22 June 1998 along with York Region chairman Bill Fisch. The plan called for an extension of Highway 404 north to Green Lane, and expansion of Highway 9 (now part of York Region Road 31), the western extension of Davis Drive outside town, to five lanes between Highway 400 and Bathurst Street. At the time over 20,000 vehicles used Davis Drive on an average day.[23]

Preliminary clearing in a field north of Green Lane in April 2011

Advanced construction work near Doane Road in June 2014

In the interim period, work went into expanding the six lane freeway through Toronto and Markham. In early 1998, the MTO announced plans for two contracts to widen Highway 404 south of Highway 7. The first contract converted the grass median into an additional lane in each direction with a central barrier between them. High-mast lighting was also installed, replacing the unique luminaires used on the freeway. The second contract resulted in an additional lane in each direction on the outside the existing freeway south of Steeles Avenue, making it ten lanes wide. The proposal to eventually convert the inner lanes into HOV lanes was announced at the same time.[24]
The next year, Highway 404 was widened to six lanes between Highway 7 and Major Mackenzie Drive.[6]

One of the three original HOV lanes in Ontario opened on Highway 404 in late 2005; the other two were the lanes on Highway 403. In early 2004, construction began on a new ramp to westbound Highway 401, curving beneath the southbound lanes of Highway 404. On 13 December 2005, the southbound HOV lane was opened to traffic.[26]
Work on the northbound HOV lane began shortly thereafter, opening at 8:30 am on Monday, 23 July 2007.[27]

The MTO formally announced plans to alleviate traffic in Newmarket on 28 August 2000: a three contract project to widen and extend Highway 404. The first contract added an additional lane in each direction in the grass median from Major Mackenzie Drive to Bloomington Road. A second contract then extended those two lanes north to Aurora Sideroad. These two projects both began in the summer of 2001 and were completed in December.[6] The third contract called for a four lane extension from Davis Drive to Green Lane and the reconstruction of Green Lane into a four-laned arterial road between Leslie Street and Woodbine Avenue.[28]
This contract began shortly after the announcement in September 2000. The extension was opened to traffic on 8 February 2002 at a ceremony attended by York North MPP Julia Munro and York Region chairman Bill Fisch.[29][30]

On 16 May 2006, the MTO announced plans to extend Highway 404 by 15 kilometres (9.3 mi) from Green Lane to Ravenshoe Road at the south end of Keswick. The first contracts were awarded later that year for the construction of the northbound bridge over Green Lane,[31]
followed by two structures over Mount Albert Road, west of Woodbine Avenue, begun in late 2008 and completed in 2009.[32][33]
By April 2011, the extension had been cleared and graded.[34]
Completion was originally scheduled for 15 December 2012 with landscaping work to continue the following spring.[35][36]
However, due to soil conditions and utility relocation issues, the project was delayed by nearly two years. The C$99 million extension opened on 17 September 2014.[37]

On 9 December 2016, it was announced that the segment between Highway 407 and Stouffville Road would be widened from six to eight lanes, and that an HOV lane would be added in each direction.[38] The project will also add a carpool lot at Major Mackenzie Drive, upgrade overpasses at Highway 7 and 16th Avenue, and replace the Rouge River overpass. The project started in 2017 and is expected to be completed in 2021.[38]

Long term proposals by the province call for Highway 404 to be extended to Highway 12, between Sunderland and Beaverton. This extension would follow a new alignment to Port Bolster, east of which the freeway would incorporate the existing two lanes of Highway 48. It has drawn criticism from various environmental groups who claim it will only serve to accelerate urban sprawl north of Toronto.[39][40]

^"Ministries at Loggerheads Over Extension of Highway". The Globe and Mail. Toronto. 26 May 1981. p. 5.

^Keating, John (3 August 1982). "404 Extension Opens Aug.10". Neighbours. The Toronto Star. p. 11. The public is cordially invited to attend the official opening of Highway 404, from Gormley Road to Bloomington Side Road on Tuesday, August 10, 1982 at 11:00 a.m. on the southbound lanes of Highway 404 at Bloomington Side Road.

^"New Stretch of 404 to Open". Neighbors North Edition. The Toronto Star. 10 September 1985. p. NR12. Highway 404 should be open to the Aurora Sideroad by the end of September, a Ministry of Transportation official said last week.... Tenders for the next and final stage of the highway from the Aurora Sideroad to Davis Drive in Newmarket will be called in December and awarded in January the spokesman said.